1. Field of the Invention
The present invention relates to a technique for pixel interpolation performed by an image processing circuit included in a digital camera or the like.
2. Description of the Background Art
Image pickup elements, such as CCDs, CMOSs and the like which are used in digital cameras and the like, perform photoelectric conversion of light received through color filters, to output pixel signals. Such color filters include RGB color filters, YMCK color filters and the like. Then, from a single-chip image pickup element, a pixel signal for one color is outputted per pixel. For example, in a case of using the RGB color filter, for one pixel, a pixel signal for one of R (Red) component, G (Green) component and B (Blue) component is outputted.
For this reason, as to the pixel signals outputted from the single-chip color image pickup element, an interpolation process has to be performed on pixel signals for other color components. Various algorithms are used to perform such an interpolation process. For example, the degrees of correlation in vertical and horizontal directions are calculated and pixel interpolation is performed by using the pixels in the direction where the degree of correlation is higher. Alternatively, weights are assigned in accordance with the respective distances between the specified pixel and the surrounding pixels and then pixel interpolation is performed.
In Japanese Patent Application Laid Open Gazette No. 2006-186965, a gray area and a color area are discriminated in an image and pixel interpolations in accordance with respective characteristics of these areas are applied to these areas. Especially, there is some contrivance to suppress occurrence of false colors in an area positioned at a boundary between the gray area and the color area. Specifically, the area at the boundary between the gray area and the color area is judged to be a gray image as to the correlation direction and the pixel interpolation for color image is applied thereto.
By using the pixel interpolation method disclosed in Japanese Patent Application Laid Open Gazette No. 2006-186965, it is possible to reduce the false colors caused by the pixel interpolation. In other words, in the area at the boundary between the gray area and the color area, paying attention to that respective pixel values of color components of RGB are approximate to one another, the correlation direction is judged by using pixels closer to the specified pixel, without distinction of RGB. On the other hand, the pixel interpolation is performed with distinction of RGB, to achieve an interpolation result with high precision.
Thus, the reason why such a contrivance for the pixel interpolation has to be performed especially in the area at the boundary between a gray image and a color image is that there is a possibility that the false colors may occur in high-frequency components such as fine lines in a horizontal or vertical direction in this area or the like.